A photochemical hole burning (PHB) effect is a phenomenon arises in an optical device when, for example, a monochromatic light is emitted to dye molecules monodispersed in a material such as polymer film or organic glass. In such a hole burning effect, a hole of a reduced absorption coefficient is created in the wavelengths of absorption spectra of the molecules. Once created, such a hole almost permanently stays in the exists in the optical device.
In other words, when the guest molecules such as the dispersed dye molecules or ions exist in the host molecules such as the organic glass or polymer film, a uniform and narrow range of lower absorption coefficient hole appears in the ununiformly dispersed optical absorption spectra of the guest molecules.
The PHB is induced by a chemical change of material in a broad sense, such as optical isomerism, ionization of guests such as dye molecules or positional change of the guests in the host molecules, caused by optical excitation. The reduction of the absorption coefficient is also arises when there is a structural change in the hosts surrounding the guests. This is called a photophysical hole burning. In this invention the PHB includes this photophysical hole burning effect.
The PHB effect has attracted attention as a principle of operation in an optical memory. Since the PHB causes a birefringence of optical beam, in a telecommunication filed, it has also attracted attention as a cause of optical polarization.
As of today, there is no specific measuring instrument to measure the PHB effects. One possible method is a saturation spectroscopy in which a high power laser is used as a pump light while a variable frequency laser such as a dye laser is used as a frequency sweeper.
The recent development of a long distance, large volume optical communication technology owes to low loss optical fibers and high gain, wide frequency range and low noise optical amplifiers. However, with the development of performance in such optical components, it has been realized that the polarization dependency in the materials of such optical components is now a significant factor that limits the further development of performance.
This polarization dependency, which deteriorates a signal to noise (S/N) ratio, is caused by a polarization dispersion, a polarization dependent loss or a polarization hole burning. Art example of the polarization hole burning in this case is a phenomenon arises when a signal light incidents to an optical fiber amplifier doped with erbium in which gains are slightly different between a direction of the signal light and a direction perpendicular to the signal light.
Under this phenomenon, since noises in a spontaneous emitted light which are perpendicular to the signal light are amplified with higher gain, the overall S/N ratio decreases. Similar problems also arise in the optical fiber. At present, there is not an appropriate measuring method to accurately analyze these problems.
Further, there is not an effective method for measuring a PHB effect to be used in a multi-wavelength ultra high density optical memory. In the measuring method of saturation spectroscopy as mentioned above, the PHB effect may change because the temperature of the host materials increases when a higher power laser light is applied to the host materials. If a lower power laser light is used, since a half width of a multiple resonance is small, a slow frequency sweep is necessary to avoid any transitional responses, which requires a longer measurement time. Further, in an optical communication field, not only to measure the PHB phenomenon in the optical devices but is also necessary to quantitatively analyze the polarization dependency induced by the PHB.